1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to synchronizing symbol timing synchronization applicable to an orthogonal frequency division multiplexing (OFDM) receiver, and more particularly, to synchronizing symbol timing synchronization applicable to an OFDM receiver that performs the symbol timing synchronization by cross-correlation between channel impulse responses.
2. Description of the Related Art
The orthogonal frequency division multiplexing (OFDM) modulation converts incoming serial data to parallel data in unit of a block, multiplexes the parallel symbols to orthogonal carrier frequencies, and thus transforms the broadband transmission to the narrowband transmission. The OFDM, one of multi-carrier modulation algorithms, shows high performance in multipath and mobile telecommunication networks, and enhances frequency utilization by use of a plurality of carriers.
A frame structure of an OFDM signal transmitted by a general OFDM system, includes an effective symbol duration and a guard interval (GI). The effective symbol duration carries data to be transmitted. The GI is used to reduce inter-symbol interference that is caused when a delayed symbol overlaps with a successive incoming symbol through the signal transmitting of the radio channel in the multipath channel environment. Additionally, the GI is used at an OFDM receiver to recover the symbol timing to prevent timing drift because of a symbol clock difference between the transmission and the reception.
FIG. 1A and FIG. 1B illustrate a conventional symbol timing synchronization apparatus applied to an OFDM receiver by use of a matched-filter. In particular, FIG. 1A is a block diagram of the conventional symbol timing synchronization apparatus applied to the OFDM receiver, and FIG. 1B is a graph showing a timing offset detected by the symbol timing synchronization apparatus of FIG. 1A.
Referring to FIG. 1A, the conventional symbol timing synchronization apparatus applied to the OFDM receiver includes an analog-to-digital converter (ADC) 10, a correlation section 20, and a detector 30.
The ADC 10 converts an incoming OFDM signal to a digital signal by performing sampling, quantization, and coding operations.
The correlation section 20 computes a correlation value between a received signal and a reference signal by multiplying and adding the received digital signal and the reference signal. The reference signal is known in advance in the OFDM transmission and reception. In practice, a complex conjugate of the reference signal is multiplied to the received signal. The correlation section 20 can be implemented by a matched-filter, a correlator, and the like.
The detector 30 detects a timing offset using the computed value of the correlation section 20. To obtain the symbol timing offset, the detector 20 detects a maximum value from multiplication values of the computed values at the correlation section 20, which are fed in sequence. Alternatively, the detector 30 can obtain the symbol timing offset by detecting a maximum value from multiplication values of the incoming signal and the delayed signal.
In FIG. 1B, the timing offset detected by the symbol timing synchronization apparatus using the matched-filter, has a large variance and low accuracy with respect to the detected timing offset. At a signal-to-noise ratio (SNR) ranging from −5 dB to 5 dB, as shown in FIG. 1B, probability of the detected timing offset lies approximately between 50% and 80%. Hence, the detected timing offset is inapplicable with respect to the low SNR. In addition, from the −5 sample to +5 sample based on a location of the effective data of the symbols, it is hard to accurately detect the timing offset in view of its probability.
FIGS. 2A and 2B illustrate a conventional symbol timing synchronization apparatus applied to the OFDM receiver using a channel impulse response. In particular, FIG. 2A is a diagram of the conventional symbol timing synchronization using the channel impulse response, and FIG. 2B is a graph showing a timing offset detected by the symbol timing synchronization apparatus of FIG. 2A.
In FIG. 2A, the symbol timing synchronization apparatus detects the timing offset by use of correlation between a time domain and a frequency domain. Here, x(n−w) is a signal delayed by the time w, X is a received signal in the frequency domain, Z is a reference signal known in advance to the frequency domain, and h(n) is the channel impulse response.
In the conventional symbol timing synchronization apparatus, a fast Fourier transformer (FFT) 50 converts an OFDM signal received in the time domain to a signal of the frequency domain. A multiplier multiplies the converted signal by a complex conjugate of the reference signal.
An inverse fast Fourier transformer (IFFT) 60 converts the multiplication value of the received signal and the reference signal in the frequency domain, to a signal of the time domain. As a result, when the received signal matches the reference signal, the output value from the IFFT 60 becomes the channel impulse response.
The output value from the IFFT 60 can be given from Equation 1 as follows.
                              h          ⁡                      (            n            )                          =                                            IFFTX              k                        ⁢                          Z              k              *                                =                                                    1                /                n                            ⁢                                                ∑                                      k                    =                    0                                                        N                    -                    1                                                  ⁢                                                      X                    k                                    ⁢                                      Z                    k                                    *                                      ⅇ                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                      π                      ⁢                                                                                          ⁢                                              kn                        /                        N                                                                                                                  =                                                            1                  /                  N                                ⁢                                                      ∑                                          k                      =                      0                                                              N                      -                      1                                                        ⁢                                                            Z                      k                                        ⁢                                          ⅇ                                              j                        ⁢                                                                                                  ⁢                        2                        ⁢                        π                        ⁢                                                                                                  ⁢                        k                        ⁢                                                                                                  ⁢                                                  w                          /                          N                                                                                      ⁢                                          Z                      k                                        *                                          ⅇ                                              j                        ⁢                                                                                                  ⁢                        2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  kn                          /                          N                                                                                                                                =                              δ                ⁡                                  (                                      n                    -                    w                                    )                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, h(n) is the channel impulse response, X is the received signal in the frequency domain, Z* is the complex conjugate of the reference signal previously known in the frequency domain, and N is the length of the received signal. It can be seen from Equation 1 that the output value of the IFFT 60 is the delta function that outputs a signal when n=w. As such, the symbol timing w can be detected by the computation of the channel impulse response at the IFFT 60.
Referring to FIG. 2B, similar to the symbol timing synchronization apparatus using the matched filter, the symbol timing synchronization apparatus using the channel impulse response computed at the IFFT has also the large variance and the low accuracy with respect to the detected timing offset. As shown, the probability of the timing offset, which is detected at the SNR ranging from −5 dB to 5 dB, lies approximately between 60% and 80% and is less sensitive to the SNR in comparison with the probability using the matched filter. However, from the −5 sample to +5 sample based on a location of the effective data of the symbols, it is difficult to accurately detect the timing offset in view of its probability.